Calcite converts to Quicklime on heating. If burned higher or too long it converts to dead burned lime. The decomposition process of this compound can generate tremendous amounts of gases and is quite complex and the subject of much discussion and research. Nilo Tozzi, an expert in tile manufacturing, says that 'calcium carbonate decomposes at about 880C when alone but when mixed into a ceramic body its decomposition starts at about 700C'. While some sources indicate a narrower range than shown here, we feel this wider range recognizes the many factors the play when calcite is found in bodies and glazes in a ceramic kiln (particle surface area, atmosphere, pressure, interactions with surrounding particles, density and thickness of the containing matrix, speed of firing). This material is highly suspect in glaze flaws related to the generation of gases from constituent materials in the glaze recipe. It may be advisable to source CaO from a frit or wollastonite if you have a problem. See the book Thermal Decomposition of Ionic Solids by Andrew K. Galwey, Michael E. Brown.
Ceramic materials, especially clays, often contain carbon and organic compounds. When they are fired in a kiln, these must burn out, often producing complications.